In current single speed bicycle drive mechanisms, the rear wheel sprocket is driven by a chain coupled to a single forward mounted drive sprocket. This drive sprocket is in turn rotated by pedals revolving around a fixed axis to which the rider applies a circular motion. As a consequence of the motion being applied in circular fashion, the torque imparted to the drive sprocket varies with the relative position of the pedal. As the rider's foot passes over top dead center in an arc towards bottom dead center, the torque applied varies from zero to a maximum and back to zero again. Thus, maximum torque is achieved only at one instantaneous position in this cycle and, when the pedal is near the top or bottom of its power arc, it contributes little to the bicycle's driving force.
An attempt to overcome the problem associated with the circular paths of the pedals is illustrated in U.S. Pat. No. 3,891,235 where the pedals move in a vertical rectilinear path. One problem associated with this design, namely coincident application of the pedaling force with respect to the chain drive, necessitates a highly complicated guide mechanism to allow the required motion. Another problem is associated with the racheting rear sprocket mechanism, specifically environmental considerations, in which dirt may adversely affect the operation of the complicated rachet means.
Another attempt to overcome the problems of conventional bicycle drives having a circular pedal path is shown in U.S. Pat. No. 3,820,820 where the pedals move in a generally oblong path with a substantially long downward or forward stroke. However, this construction features a substantial length of chain which must receive the pedal force without support from a sprocket and appears to be limited to the use of relatively small, widely spaced chain sprockets which will reduce the mechanical advantage applied to the wheel unless a separate intermediate drive sprocket is used. In addition, the design reduces ground clearance and increases overall width due the necessity for using oversize pedal stabilizers.